org.hibernate.MappingException : Unknown Entity?

This is weird but, i get this error while trying to insert or access the table through hibernate. Thing is.. i was able to insert properly in the beginning when i developed it. after a while i get this kind of error where i cant insert nor can i access the table rows. I have observed that sometimes it mapps and sometimes it doesnt(random way). Is there some property i need to set? or something like that?

PS: when i clean my project in Myeclipse it works sometimes and sometimes it doesnt. I didnt make any syntactical error .. i was able to run it . i can access at random times.